As above bigger the better or any motorbike security.

I went to a place that makes chains for cranes. They gave me a length of chain and some box section steel.

I drilled holes and bolted it to the wall. Icut the leg off an old pair of trousers and threaded the chain through to save scratching the wheel

Only money I spent was a decent padlock from B&Q

Bad men can't get to the bolts because the chain is in the way.
HTH